The present application relates to the field of radiation imaging. It finds particular application with computed-tomography (CT) scanners configured to examine an object using radiation and generate a three-dimensional image of the object from detected radiation. It also relates to other radiation systems where correcting projection data to account for changes to one or more operating parameters (e.g., source voltage, source current, temperature, etc.) may be useful.
Radiation systems (e.g., also referred to as imaging systems and/or radiation imaging systems) such as computed tomography (CT) systems, diffraction CT, single-photon emission computed tomography (SPECT) systems, projection systems, and/or line systems, for example, are used to provide information, or images, of interior aspects of an object. Generally, the object is exposed to radiation comprising photons (e.g., such as x-ray photons, gamma ray photons, etc.), and an image(s) is formed based upon the radiation absorbed and/or attenuated by interior aspects of the object, or rather an amount of photons that is able to pass through the object. Generally, highly dense aspects of the object absorb and/or attenuate more radiation than less dense aspects, and thus an aspect having a higher density, such as a bone or metal, for example, may be apparent when surrounded by less dense aspects, such as muscle or clothing.
Radiation systems can be generally be divided into two classes, single energy and multi-energy (e.g., dual-energy). Single energy systems are configured to utilize a single energy spectrum to generate an image of an object and typically provide density information associated with the object or aspects thereof. Multi-energy systems are configured to utilize two or more distinct energy spectra to generate an image of an object and typically provide additional information about the object (e.g., such as density information and z-effective information).
Measurements acquired from a radiation system can vary due to changes in operating parameters (e.g., source voltage, source current, system temperature, etc.) or operating state of the radiation system, which can affect the images produced from the examination, particularly z-effective images generated by multi-energy systems. For example, a first examination of an object may yield measurements that are different than the measurements yielded from a second examination of the same object, performed several hours later, due to temperature changes of the radiation system. This may be true even when input parameters to the radiation system (e.g., specifying a desired radiation energy output, desired dosage, etc.) are held constant. The difference between the actual operating state and a reference operating state (e.g., upon which image reconstruction constraints and/or other system constraints are based) for a given set of input parameters is referred to herein as drift, system drift, and/or the like.
System drift can interfere with the ability of a user or automated process to accurately detect an area of interest (e.g., threat item, cancer cells, etc.) within the object because the area of interest may appear different (e.g., appear to have a different density, atomic number, shape, etc.) in respective images. Accordingly, systems and techniques have been devised for correcting for drift. One such technique is disclosed in U.S. Pat. No. 7,224,763, assigned to Analogic Corporation, which is incorporated herein by reference. The technique provides for using a filter, such as a copper filter, to measure drift. Based upon this measurement, correction factors are generated and applied to image data (e.g., post image reconstruction) to correct the image(s) of the object. While such a technique has proven useful, the accuracy of the technique is dependent upon the placement of the filter, which is subject to human error. Additionally, degradations in the detector array over time may reduce the accuracy of the technique.